This invention relates to improved methods for plating metal onto a polyimide surface, and to further improvements in the quality of metal-plated polyimide substrates.
Printed circuit boards have become the dominant vehicle for mounting and interconnecting electronic components in order to manufacture a desired electronic circuit. The printed circuit board usually consists of a sheet of a dielectric substrate constructed from various filled or unfilled synthetic materials. The substrate is provided with a pattern of thin metal foil which functions as a conductive path on one or both sides. The paths or "traces" are usually formed of a conductive material such as copper, palladium, nickel or gold. The traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations.
Thermoplastic materials such as polyimides are particularly suitable for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and easy moldability. However, polyimide substrates are not easily provided with a strongly adherent metal trace. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during subsequent manufacturing steps or during use of the circuit board.
Several attempts have been made to increase the adhesion of the conductive metal traces to polyimide substrates. Adhesion is generally measured as "peel strength", i.e., the force under controlled conditions required to peel the trace from the substrate. One of the prior art methods for improving adhesion involves grit-blasting the surface to provide a roughened profile which anchors the subsequently-applied metals. Other methods call for the use of chemical swelling agents or penetrants to swell the surface.
Dissatisfaction with the above-described adhesion promotion techniques led to several new discoveries in this area of technology. As described in application Ser. No. 947,116 of Dumas et al., filed Dec. 29, 1986, now U.S. Pat. No. 4,775,449, adhesion of a metallization layer to a polyimide surface can be improved by treatment of the surface with an adhesion-promoting compound characterized by the presence of an &gt;NO-- moiety, such as hydroxylamine. Furthermore, Foust et al. teach an adhesion-promotion method utilizing organic double bond-containing compounds such as thiourea in application Ser. No. 103,618, filed Sept. 28, 1987. Both of these methods are frequently enhanced by an initial surface pretreatment which involves contact of the surface with a mild etching compound, followed by contact with a basic solution which often includes short chain alcohols, and then contact with a solution of an alcohol and another mild etching compound. This pretreatment results in a clean, water-wettable surface which is particularly suitable for the subsequent adhesion-promoting treatment.
There is a growing interest at commercial plating facilities in plating processes which do not require the use of flammable solvents such as methanol and ethanol. As in other chemical industries, attention has centered on the use of aqueous-based systems because, in addition to being nonflammable, water is nontoxic and easy to handle.
However, aqueous solutions generally do not have the solubilizing capability of most organic solvents. For example, in the pretreatment processes described above, a surface residue of plastic formed during the initial etching step is very effectively removed from the surface during the subsequent steps which use methanol or other alcohols. In contrast, water is not effective for removing this residue, thereby leaving a substrate which is not particularly amenable to the subsequent adhesion promotion and plating processes.
It is therefore a primary objective of the present invention to provide an effective aqueous-based pretreatment process for polyimide surfaces which are to be subsequently treated with an adhesion promoter and then plated with a metal.
An additional objective of this invention is to provide a method for applying highly adherent metal layers onto polyimide substrates while eliminating or greatly reducing the use of volatile solvents.
Yet another objective of the present invention is to provide a metal-plated polyimide substrate, e.g., in the form of a printed circuit board, wherein the circuit metal is characterized by a smooth finish, bright appearance, and a high level of adhesion to the polyimide surface.